1. Field of the Invention
The present invention relates to an over speed control circuit for optimizing the power output of a wind turbine generator and more particularly to a circuit for optimizing the operational time and thus power output over time of a wind turbine generator which coordinates with known over speed relay lockout protection circuitry and incorporates closed feedback control that periodically measures the output voltage of the generator to regulate its speed by electronically controlling the load on the generator to minimize activation of the over speed relay lockout protection circuitry.
2. Description of the Prior Art
Wind turbine generator systems are generally known in the art. Examples of such systems are disclosed in U.S. Pat. Nos. 4,565,929; 5,506,453; 5,907,192; 6,265,785; and 6,541,877. Such wind turbine generator systems are also described in U.S. Patent Application Publication Nos. US2002/0117861; 2005/0034937; 2005/0230979; 2005/0236839; 2006/0006658; and 2006/0012182. Due to the ever-increasing demand and increasing cost for electrical power, renewable energy sources, such as wind turbine generator systems, are becoming more and more popular for generating electrical power. Such wind turbine generator systems are known to be used individually to generate supplemental or excess power for individual, residential or light industrial users to generate electrical power in the range of 1-2 kw. Such wind turbine generator systems are also known to be aggregated together, forming a wind turbine generator farm, to produce aggregate amounts of electrical power. It is also known that unconsumed electrical power generated by wind turbine generators is connected to the utility power grid.
Such wind turbine generators are known to include a wind turbine, which includes a plurality of turbine blades connected to a rotatable shaft. The rotatable shaft is rigidly connected to a direct current (DC) generator. Wind causes rotation of the wind turbine which acts as the prime mover for a DC generator. The generator, for example, a self-excited generator, generates DC electrical power.
One problem with such systems is that wind speeds are not constant. As is known in the art, the voltage output of the generator is a cubic function of the speed of rotation of the turbine blades and the direct connected generator. As such, the effect of wind gusts on the wind turbine generator must be controlled to prevent damage to the wind turbine generator.
Some wind turbine generator systems are known to use some type of mechanical braking to protect the wind turbine generator from an over speed condition. For example, U.S. Pat. No. 5,506,453 utilizes the pitch of the wind turbine blades to protect the wind turbine from over speed. In particular, the blades of the wind turbine are mechanically coupled to a rotatable mechanical hub. The blades are configured so as to be rotatable about their longitudinal axis relative to the hub allowing the pitch of the turbine blades to be varied. The pitch of the blades is turned in such a way as to create braking of the wind turbine.
Other known systems utilize mechanical brakes, such as disclosed in U.S. Patent Application Publication No. US 2005/0034937. Yet other systems disclose the use of aerodynamic-type brakes as well as mechanical brakes, for example, as disclosed in U.S. Pat. No. 6,265,785, to protect the wind turbine from over speed.
While mechanical brakes do an adequate job of protecting the wind turbine generator from over speed, mechanical braking systems do little to optimize the operational time and thus power output of the wind turbine generator. Moreover, such mechanical braking systems are mechanically complex and are, thus, relatively expensive.
As such, electrical braking systems have been developed to control over speed of wind turbine generator systems. For example, Japanese Patent Publication JP2000-179446 discloses an electrical braking system for a wind turbine generator. The system disclosed in the Japanese patent publication includes a relay whose contacts are connected across the output terminals of the generator. When an over speed condition is detected, the relay is energized which, in turn, shorts out the output terminals of the generator, which loads the generator and causes it to slow down and stop.
In many countries, for example, in Europe, such relay protection is dictated by industrial standards, for example, the Energy Networks Association, an engineering association in the UK, promulgated Engineering Recommendation G83/1, September 2003, Recommendations For the Connection of Small-Scale Embedded Generators (Up to 16 A Per Phase) In Parallel With Low Voltage Distribution Networks”, specifies an over speed lockout relay connected across the generator terminals. Upon detection of an over speed condition, the lock out relay shorts out the generator terminals, which causes the generator to slow down and stop. The standard specifies a three-minute wait period before the relay can be de-energized so that the wind turbine generator can be restarted.
Although the electrical brake is effective in preventing damage to the wind turbine generator due to over speed, such outages frustrate the practicality of using such wind turbine generator and connecting them to a utility power grid. Thus, there is a need for a control circuit for a wind turbine generator that not only protects the wind turbine generator from over speed, but also optimizes the time that the wind turbine generator is operational and thus maximizes the output power from the generator.